Currently available human monoclonal antibodies (mAbs) which neutralize primary isolates of HIV are directed against only a few regions of the HIV envelope glycoproteins. Some primary isolates are resistant to many of these mAbs but can be neutralized by pooled sera from HIV-infected individuals, suggesting that there may be additional neutralizing epitopes which have not yet been identified. To develop new mAbs with broad and potent neutralizing activity whose activity will complement those of previously described mAbs, which can be used for passive immunization, and whose epitopes can be characterized and incorporated into the design of candidate HIV vaccines, we propose to: (a) Produce human mAbs with potent and broad neutralizing activity against HIV-1 primary isolates that target previously unidentified epitopes. Existing human anti-HIV mAbs have been selected on the basis of their binding activity. We propose to select anti-HIV mAbs based on their function, i.e., their ability to neutralize pseudovirions containing env genes of various primary isolates. (b) Delineate the breadth of the neutralizing activity of the new mAbs selected with the neutralization screen. (c) Use a variety of immunologic, biochemical, molecular and physical techniques to identify and characterize the epitopes recognized by the broadest and most potent of the mAbs. Epitope mapping techniques will include the use of an antigen fragment library, random peptide phage display libraries, epitope excision/extraction and Arg/Lys chemical modification methods, and X-ray crystallographic analyses. These studies will elucidate the nature of the epitopes recognized by the best of the neutralizing mAbs, providing critical information about the structure of epitopes that induce protective Abs. These data will also elucidate the structure of the mAb paratopes, the intermolecular interactions between epitopes and paratopes, and the molecular basis for the cross-reactivity between diverse strains of HIV. (d) Use a "focused mutagenesis" technique to engineer two selected mAbs in order to improve their potency and breadth, and subsequently determine the structure of the epitopes recognized by these engineered mAbs. The results of these studies should provide both reagents for passive immunization and data on the nature of antigenic determinants of HIV glycoproteins useful for designing immunogens that will induce broadly neutralizing Abs.